Until recently, it has been difficult to conduct biosynthetic experiments on sponge metabolites with labelled precursors. We have now solved most of the technical problems associated with such work and intend to study in detail the biosynthesis of the many unusual types of sterol structures that have been encountered during the past decade in lower marine animals. The following biosynthetic pathways will be examined using radioactively labelled sterol precursors, which will be specially synthesized for these purposes: (a) mode of biosynthesis of highly alkylated sterol side chains resulting from triple and quadruple biomethylation; (b) biosynthesis of the many novel cyclopropyl-containing sterol side chains, which occur solely in marine organisms; (c) origin of the unique 24-nor-sterols and 27-norergostanes - a group of sterols in which the terminal methyl group of cholesterol is lost; (d) biosynthesis of cyclopropene-containing side chains - another unique structural feature of sponge sterols; (e) biosynthesis of 19-nor sterols - a group of sponge sterols which in terms of their nuclei resemble the currently used oral contraceptives; (f) extent of de novo biosynthesis of sterols in lower marine animals; (g) role of symbionts and other unicellular organisms in sterol biosynthesis using intact as well as cell-free systems.